More information if it helps:
I created a new organisation, added centos7(base, centosplus, extras and updates) repos, ran a synchronise on all of them having configured variations of Mirror on Sync, immediate and ondemand download policies and nothing appears to have changed.
i.e. I still get 404 when I try to access the repo via get or yum.
These repos did work ok at one point but have stopped working now. I can’t say for certain but they may have stopped working after I tried to sync the centos errata using information from here:
Is it possible that has had an effect?